Apparatus for producing liquid nitrogen

ABSTRACT

An apparatus for producing liquid nitrogen is provided. The apparatus includes a heat exchanger, a pair of turbine-boosters, a warm compressor, an air separation unit having a single column, a top condenser and a bottom reboiler, a liquid/gas separator, and an optional subcooler. The apparatus is configured to produce merchant or non-merchant grade liquid nitrogen using the pair of turbine-boosters to provide refrigeration and energy for the process.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to an apparatus for separating air into its components. More specifically, embodiments of the present invention are related to an apparatus for producing merchant or non-merchant grade liquid nitrogen using a pair of turbo-boosters to provide refrigeration and energy for the process.

SUMMARY OF THE INVENTION

An apparatus is provided for producing nitrogen through the cryogenic separation of air. In one embodiment, the apparatus can include a heat exchanger configured to receive a main air feed comprising filtered purified and compressed air at a pressure of at least 5 bar; an air separation unit in fluid communication with a cool side of the heat exchanger, the air separation unit configured to receive cooled air from the heat exchanger and produce gaseous nitrogen and waste gaseous oxygen, wherein the air separation unit comprises a single column having a bottom reboiler and a top condenser; a recycle compressor in fluid communication with a warm side of the heat exchanger such that the recycle compressor is configured to receive a nitrogen recycle from the heat exchanger, wherein at least a portion of the nitrogen recycle is made up of gaseous nitrogen from the air separation unit; a first turbine-booster having a first booster and a first turbine, the first booster in fluid communication with the recycle compressor such that the first booster is configured to receive a compressed nitrogen recycle from the recycle compressor; a second turbine-booster having a second booster and a second turbine, the second booster in fluid communication with the first booster such that the second booster is configured to receive a boosted nitrogen from the first booster, wherein an outlet of the second booster is in fluid communication with the heat exchanger such that the boosted nitrogen from the second booster is cooled within the heat exchanger, wherein the second turbine is in fluid communication with the heat exchanger such that the second turbine is configured to receive a cooled fluid under pressure from the heat exchanger and then expand the cooled fluid to provide refrigeration for the apparatus; and a liquid/gas separator in fluid communication with the heat exchanger, the liquid/gas separator configured to receive an expanded fluid comprising nitrogen from the second booster turbine and separate the expanded fluid into a nitrogen-enriched gas and a nitrogen-enriched liquid, the liquid/gas separator in fluid communication with the second booster and the heat exchanger such that the liquid/gas separator is configured to receive a portion of the boosted nitrogen after fully cooling in the heat exchanger, the liquid/gas separator is also configured to send the nitrogen-enriched gas to the cool side of the heat exchanger.

According to other optional aspects of the invention:

-   -   the top condenser is in fluid communication with the single         column, wherein the bottom reboiler is in fluid communication         with the single column, wherein the top condenser is configured         to provide condensing duty for the single column, wherein the         bottom reboiler configured to provide reboiling duty for the         single column;     -   the apparatus includes a subcooler in fluid communication with         the single column such that the subcooler is configured to         receive a fluid from the single column that is operable to         provide subcooling for the subcooler;     -   the fluid received from the single column is an oxygen-rich         liquid from the bottom of the single column;     -   the fluid received from the single column is an oxygen-rich         liquid from a middle section of the single column;     -   the subcooler is in fluid communication with the liquid/gas         separator, the subcooler being configured to subcool the         nitrogen-enriched liquid from the liquid/gas separator;     -   the second turbine is mechanically coupled to the second         booster;     -   the first turbine is mechanically coupled to the first booster;         and     -   the recycle compressor is in fluid communication with the heat         exchanger, the bottom reboiler and the single column, such that         the recycle compressor is configured to have a fraction of         partially compressed nitrogen recycle withdrawn from an internal         stage of recycle compressor, cooled in the heat exchanger, used         by the bottom reboiler as a boiler heating fluid; and then         introduced flashed into a top portion of the single column.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it can admit to other equally effective embodiments.

FIG. 1 provides an embodiment of the present invention.

FIG. 2 provides another embodiment of the present invention.

FIG. 3 provides an additional embodiment of the present invention.

DETAILED DESCRIPTION

While the invention will be described in connection with several embodiments, it will be understood that it is not intended to limit the invention to those embodiments. On the contrary, it is intended to cover all the alternatives, modifications and equivalence as may be included within the spirit and scope of the invention defined by the appended claims.

FIG. 1 provides a basic embodiment of the present invention. Main air feed 2, which has already been purified and compressed to a pressure of about 5 to about 6 bar, is introduced to heat exchanger 10 and cooled down to a temperature near its dew point or lower to form fully cooled air feed 12. Fully cooled air feed 12 is then introduced to air separation unit 19, in order to separate the various components of air. Waste gaseous oxygen 22 is recovered from air separation unit 19 and is passed through the cold side of heat exchanger 10 in order to provide cooling to heat exchanger 10. After exiting heat exchanger 10, waste gaseous oxygen 22 can be vented to the atmosphere, used to regenerate the air adsorbers (not shown) or sent to a system of columns (not shown) if recovery of the oxygen is desired.

Gaseous nitrogen 28 is also withdrawn from air separation unit 19 and passed through the cold side of heat exchanger 10 to provide additional cooling. However, instead of venting to the atmosphere, gaseous nitrogen 28 is recycled in the process. Nitrogen recycle 38 exits air separation unit 19 and is introduced to recycle compressor 40 and compressed to form compressed nitrogen recycle 46. Compressed nitrogen recycle 46 is then cooled in second aftercooler 43 before being boosted in first booster 50 and cooled in third aftercooler 51 to form boosted nitrogen 52. Boosted nitrogen 52 is then introduced to second booster 53 in order to further compress boosted nitrogen 52 before being cooled in fourth aftercooler 55 to form fully boosted nitrogen 56. In one embodiment fully boosted nitrogen 56 can be at ambient temperature and a pressure of about 45 to about 65 bar prior to entering heat exchanger 10.

Fully boosted nitrogen 56 is then introduced to heat exchanger 10 for cooling. In one embodiment, one portion of fully boosted nitrogen 56 is fully cooled in heat exchanger 10 to form liquefied nitrogen 58, which is subsequently introduced to liquid/gas separator 60 by flashing via valve 59. In another embodiment, another portion of fully boosted nitrogen 56 is only partially cooled in heat exchanger 10 to form partially cooled boosted nitrogen 78. In one embodiment, partially cooled boosted nitrogen 78 is at or above its super critical pressure. Partially cooled boosted nitrogen 78 is then introduced into second turbine 80 in order to expand partially cooled boosted nitrogen 78 to form second expanded nitrogen 82. In one embodiment, second expanded nitrogen 82 can have a temperature that is near or below its dew point and a pressure of about 5 to about 6 bar. In one embodiment, second expanded nitrogen 82 is a two phase fluid consisting of gas and liquid phases. In a preferred embodiment, second expanded nitrogen 82 is introduced to liquid/gas separator 60 in order to separate any gaseous nitrogen from liquid nitrogen. Recovered liquid nitrogen 62 is withdrawn from liquid/gas separator 60 and collected as product. In one embodiment, gaseous nitrogen 68 is withdrawn from a top portion of liquid/gas separator 60 and combined with gaseous nitrogen 28 before introduction to the cold side of heat exchanger 10 and subsequently recycled.

In one embodiment, fraction of compressed nitrogen recycle 48 is withdrawn from compressed nitrogen recycle 46 and fed to the warm end of heat exchanger 10, where fraction of compressed nitrogen recycle 48 is partially cooled before being expanded in first turbine 70 to form first expanded nitrogen 72. In one embodiment, first expanded nitrogen 72 is reintroduced to heat exchanger 10, preferably at an intermediate point, and combined with gaseous nitrogen 28 and subsequently recycled. In one embodiment, first turbine 70 is connected by a common shaft with first booster 50 and helps to provide the energy needed for first booster 50 to compress compressed nitrogen recycle 46. Likewise, second turbine 80 is connected by a common shaft with second booster 53 and helps to provide the energy needed for second booster 53 to compress boosted nitrogen 52. In one embodiment, first turbine 70 and second turbine 80 provide substantially all of the refrigeration needs for the process.

First turbine 70 and second turbine 80 produce refrigeration by work expansion. Their respective boosters, first booster 50 and second booster 53, utilize the produced work to further compress their respective nitrogen streams.

FIG. 2 provides an alternate embodiment of the invention, which includes two recycle compressors (recycle compressor 40 and second recycle compressor 45), recycle aftercooler 41, single column 20 and subcooler 30. In one embodiment, single column 20 operates at about 5 bar. In one embodiment, nitrogen recycle 38 is partially compressed in recycle compressor 40 and cooled in recycle aftercooler 41 to form partially compressed nitrogen recycle 42. In one embodiment, partially compressed nitrogen recycle 42 has a pressure of about 8 bar. Partially compressed nitrogen recycle 42 is then further compressed in second recycle compressor 45 and cooled in second aftercooler 43 to form compressed nitrogen recycle 46, which preferably has a pressure of about 25 to about 27 bar. Fraction of partially compressed nitrogen recycle 44 is withdrawn from partially compressed nitrogen recycle 42 and fed to the warm end of heat exchanger 10. Whereas fraction of compressed nitrogen recycle 48 is only partially cooled in heat exchanger 10, fraction of partially compressed nitrogen recycle 44 is fully cooled in heat exchanger 10. In one embodiment, the split between compressed nitrogen recycle 48 and compressed nitrogen recycle 46 is about 40/60. In another embodiment, the split can be determined by balancing the needs for obtaining the desired temperature approach at the warm end of heat exchanger 10 and maintaining appropriate turbine side versus booster side flow rates based on each device's efficiencies.

After exiting the cold end of heat exchanger 10, fraction of partially compressed nitrogen recycle 44 is used to provide heat to bottom boiler 21 before being introduced via valve 93 near a top portion of single column 20. Those of ordinary skill in the art will recognize that even though recycle compressor 40 and second recycle compressor 45 are pictured as two different compressors, it is possible to use one compressor and remove fraction of partially compressed nitrogen recycle 44 from an inner stage of that single compressor.

As with all distillation columns, liquids tend to collect near the bottom, while gases rise to the top. In this embodiment, oxygen-rich liquid 24 is withdrawn from the bottom of single column 20 and introduced to subcooler 30 via valve 29. In one embodiment, oxygen-rich condensing fluid 26 is withdrawn from oxygen-rich liquid 24 and introduced via valve 35 near top condenser 23. In one embodiment, top condenser is a bath type condenser.

Gaseous nitrogen near the top of single column 20 travels up tube 27, with a portion being withdrawn as gaseous nitrogen 28 and the rest condensing within top condenser 23 before being reintroduced to single column 20. Oxygen-rich condensing fluid 26 introduced near top condenser 23 provides the needed cooling to condense the nitrogen. Waste gaseous oxygen 22 is withdrawn and used to provide refrigeration to heat exchanger 10. In one embodiment, safety purge 83 can be withdrawn as a safety precaution.

Recovered liquid nitrogen 62 is then introduced to subcooler 30 in order to further cool recovered liquid nitrogen 62 to produce liquid nitrogen product 64. Oxygen-rich liquid 24 is used to provide the necessary cooling. Any gas forming within subcooler 30 is withdrawn as oxygen-rich waste gas 34 and may be combined with waste gaseous oxygen 22 before entering the warm end of heat exchanger 10. In one embodiment not shown, oxygen-rich waste gas 34 may be warmed in heat exchanger 10 separately from waste gaseous oxygen 22 in order to allow for deeper subcooling of liquid nitrogen product 64. Oxygen purge 32 can be withdrawn from the bottom of subcooler 30 as necessary. Recycled liquid nitrogen 66 can be withdrawn from recovered liquid nitrogen 62 and introduced to the top of single column 20 as reflux via valve 36. In an optional embodiment (shown as dotted line 66 a), recycled liquid nitrogen 66 can originate from liquefied nitrogen 58 via line 66 a.

FIG. 3 provides an additional embodiment of the invention. FIG. 3 is similar to FIG. 2, except that FIG. 3 provides for increased safety by reducing the risk of hydrocarbons concentrating in subcooler 30. In this embodiment, oxygen-rich fluid 25 is withdrawn from single column 20 at a point above the bottom portion of single column 20 and introduced to subcooler 30 instead of using the oxygen-rich liquid from the bottom of single column 20. Essentially, instead of sending a portion of the bottoms liquid to subcooler 30, all of the withdrawn bottoms liquid is sent to top condenser 23 as oxygen-rich condensing fluid 26 via valve 35. Recycled liquid nitrogen 67 can be withdrawn from liquefied nitrogen 58 and introduced to the top of single column 20 as reflux via valve 36.

In certain embodiment, the feed gas to the single column is air, as opposed to a feed gas having a concentration having higher nitrogen content. The single column has both a bottom reboiler and a top condenser, and in certain embodiments, the reboiler is driven by gaseous nitrogen withdrawn from the recycle compressor, preferably at a first stage discharge of the recycle compressor. In another embodiment, the single column can be partly refluxed with liquid nitrogen split-off from a Joule-Thompson stream (e.g., high pressure nitrogen stream exiting the cool end of the heat exchanger such as stream 58). In another embodiment, column bottoms may be split for both product subcooling and for driving the top condenser, or all of column bottoms can used for driving the top condenser with product subcooling being done via an oxygen-rich liquid column sidedraw stream.

While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, language referring to order, such as first and second, should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps or devices can be combined into a single step/device.

The singular forms “a”, “an”, and “the” include plural referents, unless the context clearly dictates otherwise.

Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range. 

I claim:
 1. An apparatus for producing nitrogen through the cryogenic separation of air, the apparatus comprising: a heat exchanger configured to receive a main air feed comprising purified and compressed air at a pressure of at least 5 bar; an air separation unit in fluid communication with a cool side of the heat exchanger, the air separation unit configured to receive cooled air from the heat exchanger and produce gaseous nitrogen and waste oxygen, wherein the air separation unit comprises a single column having a bottom reboiler and a top condenser; a recycle compressor in fluid communication with a warm side of the heat exchanger such that the recycle compressor is configured to receive a nitrogen recycle from the heat exchanger, wherein at least a portion of the nitrogen recycle is made up of gaseous nitrogen from the air separation unit; a first turbine-booster having a first booster and a first turbine, the first booster in fluid communication with the recycle compressor such that the first booster is configured to receive a compressed nitrogen recycle from the recycle compressor; a second turbine-booster having a second booster and a second turbine, the second booster in fluid communication with the first booster such that the second booster is configured to receive a boosted nitrogen from the first booster, wherein an outlet of the second booster is in fluid communication with the heat exchanger such that the boosted nitrogen from the second booster is cooled within the heat exchanger, wherein the second turbine is in fluid communication with the heat exchanger such that the second turbine is configured to receive a cooled fluid under pressure from the heat exchanger and then expand the cooled fluid to provide refrigeration for the apparatus; and a liquid/gas separator in fluid communication with the heat exchanger, the liquid/gas separator configured to receive an expanded fluid comprising nitrogen from the second turbine and separate the expanded fluid into a nitrogen-enriched gas and a nitrogen-enriched liquid, the liquid/gas separator in fluid communication with the second booster and the heat exchanger such that the liquid/gas separator is configured to receive a portion of the boosted nitrogen after fully cooling in the heat exchanger, the liquid/gas separator is also configured to send the nitrogen-enriched gas to the cool side of the heat exchanger.
 2. The apparatus as claimed in claim 1, wherein the top condenser is in fluid communication with the single column, wherein the bottom reboiler is in fluid communication with the single column, wherein the top condenser is configured to provide condensing duty for the single column, wherein the bottom reboiler configured to provide reboiling duty for the single column.
 3. The apparatus as claimed in claim 1, wherein the subcooler is in fluid communication with the single column such that the subcooler is configured to receive a fluid from the single column that is operable to provide subcooling for the subcooler.
 4. The apparatus as claimed in claim 3, wherein the fluid received from the single column is an oxygen-rich liquid from a bottom of the single column.
 5. The apparatus as claimed in claim 3, wherein the fluid received from the single column is an oxygen-rich liquid from a middle section of the single column.
 6. The apparatus as claimed in claim 3, wherein the subcooler is in fluid communication with the liquid/gas separator, the subcooler being configured to subcool the nitrogen-enriched liquid from the liquid/gas separator.
 7. The apparatus as claimed in claim 1, wherein the second turbine is mechanically coupled to the second booster.
 8. The apparatus as claimed in claim 1, wherein the first turbine is mechanically coupled to the first booster.
 9. The apparatus as claimed in claim 1, wherein the recycle compressor is in fluid communication with the heat exchanger, the bottom reboiler and the single column, such that the recycle compressor is configured to have a fraction of partially compressed nitrogen recycle withdrawn from an internal stage of recycle compressor, cooled in the heat exchanger, used by the bottom reboiler as a heating fluid; and then flashed into a top portion of the single column. 